The present invention relates to improvements in flexible fastener zipper strips of the type having projecting ribs and grooves shaped to interlock with each other when pressed together and to release when pulled apart.
More particularly, the invention relates to continuous plastic strips of the type which can be manufactured at high speed by extruding the plastic fastener strips out of extrusion heads. The strips can be formed with the web so as to be attached to adjoining sheets to be joined or can be integral with the sheets. In this type of fastening strip, each strip is manufactured with ribs and grooves of the same size and shape thus making it possible to extrude only a single strip out of a single extrusion head with the strip matched to itself for interlocking. Each strip has a plurality of ribs and a plurality of grooves with the ribs sized and shaped to interlock in the grooves when the strips are pressed together.
With a construction which embodies a plurality of ribs and a plurality of grooves in each strip of the same size and shape, it is possible that when the strips are to be joined, the ribs and grooves can be improperly oriented so that the ribs will enter the wrong grooves and an insecure or improper attachment will be made. If the attachment is such that not all of the ribs are interlocked into all of the grooves, a connection of inadequate strength will result and perhaps a connection which will leak or at least not offer adequate sealing. Because the ribs and grooves are quite small, it is easy for an operator to inadvertently push the wrong rib into the wrong groove which results in an improper closure. In an example which is illustrated in the drawings, strips are shown with two ribs and two grooves, and it is quite possible without the features of the present invention for an operator to push the outermost rib into the outermost groove so that only one rib and one groove interlock and the innermost ribs and grooves do not engage. This would result in an improper inadequate closure between the fastener strips.